Communications networks often include monitoring and/or testing equipment that is distributed in different parts of the network. Examples of such network monitoring equipment include those described in U.S. Pat. No. 7,336,673 issued Feb. 26, 2008 to Ilnicki et al entitled Creating a Low Bandwidth Channel Within a High Bandwidth Packet Stream, U.S. Pat. No. 7,868,780 issued Jan. 11, 2011 to Engel et al entitled System and Method for Test Probe Management, and U.S. Pat. No. 8,427,966 issued Apr. 23, 2013 in the name of Ilnicki, entitled Programmable Passive Probe, which are incorporated herein by reference.
Often various monitoring and testing devices located at different network sites are required to coordinate their operation, so that their internal clocks must be synchronized in time. One possible mechanism for such synchronization is provided by a Precision Time Protocol (PTP) standard, which is a general protocol that was developed to synchronize clocks throughout a computer network. PTP was originally defined in the IEEE 1588-2002 standard, entitled “Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems” and published in 2002. In 2008 a revised standard, IEEE 1588-2008 was released, also known as PTP Version 2, which is incorporated herein by reference; it is not backwards compatible with the original 2002 version.
The IEEE 1588 standard defines a hierarchical master-slave architecture for clock distribution, in which a slave clock periodically adjusts its time to that of a master clock by exchanging specific PTP messages therewith. U.S. patent application Ser. No. 13/714,174, which is incorporated herein by reference, discloses a variation of the conventional PTP process wherein the master-slave timing synchronization is accomplished with fewer messages than the conventional 1588 process.
An object of this invention is to provide a mechanism for correlating network packet timestamps to a remote master clock for network packets that were generated by a network device having a free running clock.